An optical beam may be characterized with amplitude, wavelength, phase, and polarization. In an electro-optic modulator, one or more of these parameters are affected, or modulated, by an electrical signal. Accordingly, if the electrical signal modulates the amplitude of the optical beam, one may speak of amplitude modulation. Similarly, if the electrical signal modulates the phase of the optical beam, one may speak of phase modulation. To effect such modulation, one may exploit nonlinear properties of certain materials, such as lithium niobate (LNB), which changes its optical properties according to the applied electric field. In particular, the application of electric field modifies the refractive index of LNB. The phenomenon of changing the refractive index proportionally to the strength of the applied electric field is known as Pockels effect. Modulators based on Pockels effect, including those made of LNB, are in wide-spread use in, e.g., long-haul telecommunication that uses optical beams guided in optical fibers to convey information in digital form. Such modulators are also used in the field of radio-frequency (RF) photonics to impose analog RF signals onto optical beams for transmission and/or processing. In these applications, the optical beam of a specific wavelength and a specific polarization may be confined to a single-mode waveguide when traversing the modulator and being subject to external RF signal for modulation. When a single straight waveguide patterned in LNB is used, phase modulation of the optical beam may ensue. By properly configuring the waveguides in LNB, e.g. in the form of a Mach-Zehnder interferometer, where the phase in one or both arms is modulated by external signals, amplitude modulation may be achieved. Nesting and/or concatenation of the Mach-Zehnder modulators and/or phase modulators may enable arbitrary phase-amplitude modulation.
However, commonly, at the lowest level, a single electrical signal modulates a single optical beam having a fixed polarization and propagating in a single-mode waveguide patterned in electro-optic material such as LNB.